A taper round thread, a taper buttless thread or the like is provided on, for example, an oil well pipe thread hitherto according to API (Americal Petroleum Institute) Standards, such tapered screw for oil well pipes is subjected to a fitting inspection on an API thread gauge according to the above-mentioned API Standards, thereby controlling and assuring quality. A high precision will particularly be ensured on the oil well pipe thread for its purpose, a measuring item of the tapered threads then comes in taper, lead, depth, angle, coincidence with shaft center, stand-off and others, and the basic standoff on a thread gauge must come exactly within a tolerance of the Standards after all. The stand-off is then expressed such that an effective diameter of thread is predetermined for its tolerance so as to allow an interchangeability between pipes, a thread gauge having a basic diameter of thread is used for measurement of the diameter of thread, the gauge is engaged with a measuring object, the diameter of thread is then decided for dimension according to a distance between a reference plane of the thread gauge and a reference plane of the measuring object, and the distance is called stand-off.
Such tapered thread fitting inspection is carried out currently on a hand tight system in the world, and, for example, a tapered external thread gauge 1 with a tapered external thread 2 provided on the outer periphery is engaged, as shown in FIG. 1, with a tapered internal thread coupling 4 with a tapered internal thread 5 provided on the inner periphery by means of a handle arm 3 of predetermined length as aligning shaft centers of both the two carefully so as not to keep the two inclined relatively and also not to mar the tapered thread surface, and whether or not a stand-off value at the time of ending the clamping on a predetermined torque (a distance between a portion 6 whereat the tapered external thread 2 of the tapered external thread gauge 1 is engaged perfectly and a top end surface 7 of the tapered internal thread coupling 4 in FIG. 1) comes within allowable limits determined separately is measured visually and so recorded. However, if a crest 8 of the tapered external thread 2 comes in contact with a crest 9 of the tapered internal thread 5 in this case, then a so-called "biting phenomenon" or "galling phenomenon" may take place to hinder a smooth engagement, and the phenomena are apt particularly to occur where both the two threads are buttless threads. In case "biting phenomenon" or the like has taken place, the contact state is cleared by reversely rotating the tapered external thread gauge 1 as giving a slight shock or likewise thereto, a similar operation is repeated carefully so as not to incur such "biting phenomenon" or the like, a portion whereat both the two start for engagement is detected sensuously, then an engaging work of both the two must be recommenced after the tapered external thread gauge 1 is rotated to descent. Hence, a skill is somewhat required for this kind of tapered screw member fitting inspection and engaging work. Moreover, a considerably long time is required for setting work at site to inefficiency, and the tapered thread surface is capable of being damaged to an unavoidable defect.